A self-lock bolt with a slit formed along the crest portion of the thread and a method of producing the same are known, as described in, e.g., Japanese Patent Publication No. Hei 2-58016. In the bolt proposed in Japanese Patent Publication No. Hei 2-58016 is, the flanks of the bolt are pressed against the slit by the flanks of the nut so as to cause elastic deformation, and the bolt is kept from loosening by the pressing force of the elastic deformation. According to the method of producing such a self-lock bolt, a double-threaded screw having half a desired pitch is first formed by rolling, and the screw is then formed into a single-threaded screw having the desired pitch by rolling, thereby forming a slit along the crest portion of the thread.
When a slit is formed along the crest portion of the thread in this manner by a die similar to the one which is used for the roll forming of an ordinary screw, it is not always possible to produce a slit having an adequate depth. As a result, the rigidity on both sides of the slit becomes large and the amount of elastic deformation of the flanks becomes small, so that it is impossible to have an adequate locking effect (spring back effect) of the screw caused by the reaction of the elastic deformation.
To solve this problem, the present applicant proposed a method of producing a screw described in Japanese Patent Laid-Open No. Hei 7-139537. According to this method, in a first roll forming process, a thread groove having a small depth and a thread groove having a large depth are alternately arranged, and the thread groove having a large depth is formed into a slit in a second roll forming process. In this manner, it is possible to form a slit having an adequate depth.
However, the method of producing a screw disclosed in Japanese Patent Laid-Open No. Hei 7-139537 requires a special roll forming die for forming a thread groove having a small depth and a thread groove having a large depth which are alternately arranged, so that the cost of producing a roll forming die is disadvantageously increased. In addition, in the roll forming of a screw by such a die, since it is difficult to apply a uniform roll forming pressure, the production of a screw becomes difficult. Furthermore, since the depth of the slit becomes smaller than that produced by a conventional method, if the phase adjustment of the roll forming die and the groove of a screw is reversed, it is necessary to pay close attention to the phase adjustment. In this manner, the first roll forming process and the second roll forming process require skill of a high level and dexterity. Furthermore, the product yield is poor.
Accordingly, it is an object of the present invention to provide a self-lock screw with the thread having a broad root so as to form a deep slit without the need for skill of a high level or dexterity and facilitate the elastic deformation of the flanks which heightens the self-lock effect.